My girlfriend had a 10 year old rat terrier that we both raised from a puppy. She was nice to everyone, listened, happy dog. She started having issues with her white cells dropping. They took tests and found out it was cancer but couldn't figure out where. They asked us about putting her to sleep but my girl said she didn't want to until the day she started having problems to where she couldn't live normally. We spent around 4k to get the dog better and she seemed better at times but then the day came where she was lethargic only to shit and she ended up falling onto her own waste. We knew it was time. It was a painful experience for us both.
A few months later her feral cat was acting very lethargic and had a problem with his mouth, like drooling. Took him to the vet and his kidneys were pretty much gone. The doctor told us that his life would not be good at all and we would have to do a manual type of dialasis type thing injecting the cat with needles twice a day or something along those lines. He suggested putting the cat to sleep and we did as we were told life would not be fun for the cat at all.
For what its worth, when you put your animals to sleep the injection seems to be painless and they pass very very quickly. A minute or two passed after the shots were given. We stayed and pet both animals and they peacefully went to sleep in our arms.
This is a heartbreaking short about a man having to put his best friend to sleep. You can get a gist of what its like if you choose to watch.
I wish you the best.